1. Field of the Invention
This invention relates to improvements in a fuel injection control system for an engine, and more particularly to a technique for improving engine starting without degrading exhaust gas characteristics.
2. Description of the Prior Art
Most internal combustion engines are equipped with a fuel injector valve(s) for injecting fuel into the engine under a fuel injection control executed by a fuel injection control system. In general, in the fuel injection control at engine starting, a larger amount of fuel is injected than in normal engine operation to improve a starting characteristics of the engine. In other words, a fuel injection control manner suitable for the engine starting is executed when a starter switch is kept turned ON on the assumption that the turning-ON state of the starter switch corresponds to an engine starting condition. This fuel injection control manner is shifted to another fuel injection control manner suitable for the normal engine operation when the starter switch is turned OFF.
However, difficulties have been encountered in the above-discussed conventional fuel injection control, as set forth below. The above fuel injection control is intended to form an air-fuel mixture necessary for causing an initial exposition in an engine cylinder, within a short period of time. Accordingly, the fuel injection control manner for the engine starting is continued even after occurrence of the initial explosion until the starter switch is turned OFF, so that the fuel injection amount within this time duration is excessive thereby degrading the characteristics of exhaust gas.
Besides, in the conventional fuel injection control in which fuel injection is intermittently made in synchronism with or in relation to engine speed during normal engine operation, such engine speed-synchronized fuel injection is made even at engine starting. However, in this control, fuel is unavoidably concentrically injected at a time since a cycle time of engine rotation is large during engine cranking at which engine speed is very low (not higher than about 200 rpm). Additionally, at such a very low engine speed, an air flow speed in the air intake passage is very low and therefore most fuel that cannot be vaporized tends to adhere to the inner wall of the air intake passage forming a fuel flow on the air intake passage inner wall.
Such a tendency is strong particularly under a low temperature condition in which fuel vaporization is degraded. Additionally, in a system having a large volume in an intake system between the fuel injector valve and the engine combustion chamber, there is a tendency for the air-fuel mixture formed to have a nonuniformity in lean and rich characteristics. As a result, a large amount of useless fuel is injected at the engine starting thereby degrading exhaust gas purifying characteristics and engine starting characteristics.